A solid oxide fuel cell (SOFC) comprises an air electrode (cathode), a fuel electrode (anode) and an electrolyte, wherein the electrolyte is the most important component which influences the performance of the SOFC.
The electrolyte used in an SOFC is sintered at a high temperature of 1000 to 1400° C. so as to form a dense microstructure that prevents the mixing of oxygen and fuel. An SOFC is generally operated at a high temperature of 800° C. or more to maintain the oxygen ion conductivity at a high level. For this reason, there exists the problem that the materials for SOFC components are limited because of high production costs and gradual performance degradation under such a high temperature condition.
Recently, there have been developed SOFCs which can be operated at a temperature lower than 800° C. For such SOFCs, it is required to develop an electrolyte system which exhibits excellent phase stability and structural stability at 500 to 800° C. as well as a satisfactorily good oxygen ion conductivity. In order to develop such an electrolyte, novel electrolyte materials having high ion conductivity at said temperature range have been developed. On the other hand, efforts to decrease the resistance of conventional electrolyte materials, e.g. yttria-stabilized zirconia (YSZ), have been intensively attempted by making them in the form of a thin film.
In order to decrease the operation temperature of an SOFC by using a thin film form of an electrolyte, there have been conducted various studies to prepare a ceramic thin film suitable for use as an electrolyte for an SOFC by a gas-phase process such as sputtering and ion plating (see Japanese Patent Publication Nos. 2000-62077, 2000-329729 and 2004-87490.) However, such gas-phase processes are problematic in that the use of high-cost equipments and starting materials is required, the shape and size of the substrate are limited, the formation of a film is slow, and it is difficult to form a pinhole-free film.
When an electrolyte thin film is formed by a conventional powder-based process, a micron-size starting powder is used, and, when it is sintered at a high temperature of 1200° C. or more, a film composed of micron-size particles is obtained. However, when the thickness of the electrolyte thin film is smaller than the particle size of the powder, the electrolyte thin film disintegrates during grain growth, which makes it difficult to form a dense thin film.
Therefore, there has been a need to develop a cost-effective method of preparing an electrolyte thin film for SOFCs, which can maintain a stable phase, a durable structure, and satisfactory density to prevent gas permeation therethrough during the operation thereof at a temperature lower than 800° C.